1. Field of the Invention
This invention relates to the field of wafer processing. In particular, the invention relates to scheduling techniques for wafer cluster tools.
2. Description of the Related Art
In the process of manufacturing a semiconductor device such as an integrated circuit, numerous steps of micro-fabrication are performed to form a device. These steps are performed serially on the individual items of manufacture in individual modules; the items of manufacture are transferred between modules by transport mechanisms such as robots. In order to achieve desirable throughput, reliability, and fabrication quality, several conditions must be met:                1) The delivery and removal of the substrate to and from the process modules, as well as the transportation of the wafer between modules, must be accomplished in a timely manner. This timely delivery and removal of substrate is achieved when the flow of substrate is maintained in a periodic and synchronized manner. If periodicity and synchronization are not maintained, the process results will be inconsistent from substrate to substrate, and the expected throughput may be reduced.        2) It is desirable to transport the substrate in similar process flow paths to avoid inconsistency in process results due to variations in the process history of the substrates.        3) It is imperative to ensure that the articles of manufacture do not spend any pre-process or post-process time idling in modules where critical processes are performed. The addition of pre-process or post-process time in these modules degrades not only the throughput but also the process results. For example, in an IC fabrication system, if a substrate is not immediately transferred from the spin coat module to a bake module to thermally cure a photo-resist film layer, the resulting film thickness will be unpredictable. If it is impossible to totally eliminate pre-process and/or post-process times, they should be rendered as brief as possible, and any variations in these times cannot be allowed.        
The inability to meet any or all of the above conditions come from the failure to resolve transport conflicts. Conflicts are situations wherein separate modules demand a robot within a time span insufficient for the robot to service these modules
One conventional solution to the concerns listed above is the addition of extra process modules and transportation resources. However, the size limitations and geometrical constraints of a track system limit the possibility of resolving the above difficulties by adding additional process modules or transportation resources.
The addition of dedicated transfer arms to transfer substrates between adjacent modules (hereinafter called Inter Bay Transfer Arms, or IBTAs) is another method used to improve throughput and eliminate some of the pre-process and/or post-process times. However, the addition of IBTAs also has serious drawbacks. Dedicated transfer arms complicate the tool and increase its cost, constrain the position of the modules, and cannot be used everywhere in the tool. As a result, the tasks of managing the substrate flow in the track system while maintaining both high throughput and quality and resolving all transport conflicts become unmanageable.
Another conventional solution is to assign a set of substrate transport priority rules. Prior to any robot move, the control system, also referred to as the software scheduler, verifies the status of substrates in different modules and makes transfer priority decisions based on these rules. However, to achieve high throughputs, the scheduler may generate undesirable, unpredictable and variable pre-process and post-process times in critical modules, and the substrates may also be forced to follow different flow paths to complete their process cycle.
Yet another conventional solution is to add multiple grippers to robot, to enable wafer swapping. Multiple gripper robots are frequently used in the prior art in order to allow wafer swapping and minimize the number of robot transfers. However, this technique adds an additional constraint to the cluster tool, as a robot with dual grippers requires a wafer on one of the grippers in order to perform the swapping. This constrains the performance of the cluster tool. Furthermore, the swapping increases transport time between modules, which may also be deleterious to the performance of the cluster tool.
Heretofore, the requirements of conflict resolution, synchronization, quality, and path consistency referred to above have not been fully met. What is needed is a solution that simultaneously addresses all of these requirements.